Single electron reader opens path for quantum computing

September 27, 2010

Scanning electron micrograph of metallic electrodes on silicon oxide. The electrodes are isolated from each other so that there is no electric current flowing through them. A schematic illustration has been added to the figure representing the electron layer induced below the silicon oxide (source and drain) together with so-called quantum dot (SET island) which works as a charge detector. Furthermore, the dashed blue line shows a region where phosphorus donors have been placed in the silicon with the magnetic moment of the outermost electron pointing either up or down. The energy of spin-up state is higher than the energy of spin-down state in magnetic field. By controlling the voltage on a nearby plunger gate, the system can be brought at will into a working point where spin-up electron has enough energy to tunnel into the charge detector but the spin-down state of the same electron remains bound to the phosphorus. The detector is very sensitive to changes in the charge state of the phosphorus yielding noticeable current (ISET) after the spin-up electron moves. Thus the spin state of the electron can be measure by a single shot at any chosen time.

Researchers from University of New South Wales (Australia), University of Melbourne (Australia), and Aalto University (Finland) have succeeded in demonstrating a high-fidelity detection scheme for the magnetic state of a single electron, that is, the spin. The research results have just been published in Nature.

A team led by UNSW engineers and physicists have developed one of the key building blocks needed to make a quantum computer using silicon: a "single electron reader".

Quantum computers promise exponential increases in processing speed over today’s computers through their use of the “spin”, or magnetic orientation, of individual electrons to represent data in their calculations.

In order to employ electron spin, the quantum computer needs both a way of changing the spin state (write) and of measuring that change (read) to form a qubit - the equivalent of the bits in a conventional computer.

In creating the single electron reader, a team of engineers and physicists led by Dr Andrea Morello and Professor Andrew Dzurak, of the School of Electrical Engineering and Telecommunications at UNSW, has for the first time made possible the measurement of the spin of one electron in silicon in a single shot experiment. The team also includes researchers from the University of Melbourne and Aalto University in Finland.

“Our device detects the spin state of a single electron in a single phosphorus atom implanted in a block of silicon. The spin state of the electron controls the flow of electrons in a nearby circuit,” said Dr Morello, the lead author of the paper, Single-shot readout of an electron spin in silicon.

“Until this experiment, no-one had actually measured the spin of a single electron in silicon in a single-shot experiment.”

By using silicon—the foundation material of conventional computers—rather than light or the esoteric materials and approaches being pursued by other researchers, the device opens the way to constructing a simpler quantum computer, scalable and amenable to mass-production.

The team has built on a body of research that has put Australia at forefront of the race to construct a working quantum computer. In 1998 Bruce Kane, then at UNSW, outlined in Nature the concept for a silicon-based quantum computer, in which the qubits are defined by single phosphorus atoms in an otherwise ultra-pure silicon chip. The new device brings his vision closer.

“We expect quantum computers will be able to perform certain tasks much faster than normal computers, such as searching databases, modelling complex molecules or developing new drugs,” says co-author Prof Andrew Dzurak. “They could also crack most modern forms of encryption.”

“After a decade of work trying to build this type of single atom qubit device, this is a very special moment.”

Now the team has created a single electron reader, they are working to quickly complete a single electron writer and combine the two. Then they will combine pairs of these devices to create a 2-bit logic gate - the basic processing unit of a quantum computer.

Artist’s impression of a phosphorus atom (red sphere surrounded by a blue electron cloud, with spin) coupled to a silicon single-electron transistor, to achieve single-shot readout of the phosphorus electron spin. Credit: William Algar-Chuklin, College of Fine Arts, UNSW.

Qubits replace bits in a quantum computer

The qubit is the abstract quantum counterpart of the classical bit 0 or 1 in the computers today. The spin of an electron is an ideal candidate for a qubit since it has exactly two independent states. The difference to a classical bit is that the qubit can be simultaneously in states 0 and 1 rendering it conceptually more general.

To achieve the envisioned quantum computing, one has to be able to initialize and measure the states of the qubits, rotate the single-qubit states arbitrarily, and to make a certain operation involving at least two neighboring qubits—all this with high fidelity. In the work reported now in Nature, the researchers were able to initialize the spin and measure it with 92% fidelity. Thus only single spin rotations and controllable spin-spin interactions are left for the future research. After achieving these goals, the demonstration of large-scale quantum computer would be at hand.

Researchers who hope to create quantum computers are currently investigating various methods to store data. Nitrogen atoms embedded in diamond show promise for encoding quantum bits (qubits), but the process of reading the ...

University of California scientists working at Los Alamos National Laboratory and at the University of California, Los Angeles have demonstrated the ability to detect the spin of a single electron in a standard silicon transistor. ...

(PhysOrg.com) -- Researchers from Helsinki University of Technology (Finland), University of New South Wales (Australia), and University of Melbourne (Australia) have succeeded in building a working transistor, whose active ...

(PhysOrg.com) -- While researchers have already demonstrated the building blocks for few-bit quantum computers, scaling these systems up to large quantum computers remains a challenge. One of the biggest problems is developing ...

(PhysOrg.com) -- While quantum dots have existed since the 1980s, only in the past decade have physicists successfully created lateral few-electron single quantum dots. These quantum dots enable physicists to manipulate quantum ...

Recommended for you

(Phys.org)—A team of researchers from the University of Birmingham, Sorbonne Universités and Unilever Research & Development Port Sunlight, has found that human fingertips behave differently when touching something depending ...

The potential for photon entanglement in quantum computing and communications has been known for decades. One of the issues impeding its immediate application is the fact that many photon entanglement platforms do not operate ...

A major issue facing ITER, the international tokamak under construction in France that will be the first magnetic fusion device to produce net energy, is whether the crucial divertor plates that will exhaust waste heat from ...

Mechanical metamaterials, which exhibit unusual properties such as shape morphing and programmability, have been found to display further surprising features. When the materials are a step in size larger, new rules seem to ...

Controlling electronic current is essential to modern electronics, as data and signals are transferred by streams of electrons controlled at high speed. Demands on transmission speeds are also increasing as technology develops. ...

Freshwater planarians, found around the world and commonly known as "flatworms," are famous for their regenerative prowess. Through a process called "fission," planarians can reproduce asexually by simply tearing themselves ...